Optimized source for x‐ray lithography of small area devices

The design and performance of an optimized soft‐x‐ray source using an air‐cooled Al target are described in this paper. The technical considerations leading to the choice of mask material, source wavelength, vacuum enclosure, and electron optics are also described. Although the authors have previously shown the need for large, powerful x‐ray sources, the source described here is intended for exposure of small‐area surface‐acoustic‐wave (SAW) devices with submicrometer dimensions. Since distortion over large areas need not be controlled, the x‐ray source is limited to 1 mm in diameter and the electron beam power to 250 W. A 25‐μm‐thick Be vacuum window was developed which made it possible to use the Al Kα x‐ray wavelength with a thin Si mask and yet maintain the exposure area in He at atmospheric pressure. The custom‐designed electron gun, exposure monitor, and thermal control system are also described. Results are reported on the replication of a SAW device pattern in PMMA resist with 0.5‐μm linewidths.